The incubation of bacterial cultures is a standard laboratory technique in numerous biological studies. Monitoring the growth of these cultures in a shaker bath or fermenter currently requires frequent manual sampling. The work on this project has been aimed at developing a system capable of continuously monitoring growth in situ using optical measurement techniques. To date the work has demonstrated the feasibility of making these measurements in shaker flasks employing an immersed detection system and yielded a prototype system for this application. Additional efforts have been directed at designing a system which will operate in a commercial type fermenter. The principal problem of eliminating gas bubbles in the optical path of the immersed probe has been solved. The shaker flask probe consist of a light emitting diode and a photodetector mounted a right angles to one another in a fixed submersible head. Readings are taken at intervals of one minute under control of a computer system which also collects and process data. The fermenter probe is of a similar design except that the optics are kept in a bubble free region of the housing.